Process for the separation of heavy metals



Jan. Z7, 1959 J. GOFMAN ET AL PROCESS F'OR THE SEPARATION OF HEAVY METALS Filed Nov. 30, 1944 FIEL 2 Sheets-Sheet 1 Jan. 27, 1959 E J. w. GO'FMAN ET AL 2,87L25 PROCESS FOR THE SEPARATION 0E HEAVY METALS Filed Nov. 50, 1944 I 2 Sheets-Sheet 2 A sauger Patented Jan. 27,

PROCESS FOR THE SEPARATION F HEAVY METALS .lohn W. Gaiman and Robert E. Courriels, Berkeley, Calif., and Arthur C. Wahl, Los Alamos, N. Mex., assignors to the United States of America as represented by the United States Atomic Energy Commission Application November Si), 1944, Serial No. 565,989 Claims. (Cl. 260--429.1)

This invention relates to the separation of element 94 from other substances, and more particularly, to the separation of element 94 from various substances of the kind vpresent in neutron irradiated uranium, such as uranium, ssion products, and the like.

It is an object of the invention to obtain element 94 in a concentrated state by a process in which advantage is taken of the solubility characteristics of element 94 and the substances from which it is to be separated, particularly with respect to diierent oxidation states of element 94. provide a process suitably adapted for concentrations of element 94 such as are obtained from neutron irradiated uranium where foreign substances such as uranium, tission products, and the like are present. A further object is to provide a method by which plutonium may be removed from a solution by precipitation of a complex salt of sodium or similar alkali metal, acetic acid and uranium or plutonium. Other objects and advantages will be apparent from the following detailed description.

As described herein, the isotope of element 93 having a mass of 239 is referred to as 93239 and the isotope of element 94 having a mass of 239 is referred to as 94239. Element 94 may also be spoken of as plutonium, symbol Pu.

Neutron irradiated uranium may be prepared by reacting uranium with neutrons from any conventional neutron source, but preferably the neutrons used are obtained from a chain reaction of neutrons with uranium.

Sodium plutonyl acetate precipitated by the process herein described forms as a pink crystalline precipitate of low solubility in dilute Weak acid solution but which is soluble or at least decomposable in strong acids Such as nitric acid. The crystals of sodium plutonyl acetate are isotropic Vtetrahedral crystals which measure about 1 to l0 microns from one edge to the opposite point. The solubility of sodium plutonyl acetate decreases with increased sodium ion and in general is less soluble than sodium uranyl acetate.

In the drawings, Fig. l diagrammatically represents a process embodying the invention, and Fig. 2 similarly represents a modified process.

Neutron irradiation of uranium produces 92233 which has a half-life of 23 minutes and by beta decay becomes 93239. This element has a half-life of 2.3 days and by beta decay becomes 94239. Neutron irradiated uranium contains 93239, 94239 and a large number of radioactive lission products produced by reaction of neutrons on issionable atoms, such as 'U235 which is present in uranium from natural sources. It also contains minor amounts of products of the natural radioactive decay of uranium isotopes, such as UXl and UXZ. Fthe amount of 93239 and 94239 combined is generally minute, such as, for example, approximately 0.62 percent by Weight. lBy storing the neutron irradiated ura- It is a further object of the invention to n' niurn for a suitable period of time, the 93239 is converted almost entirely to 94233. The ission products are present in the neutron irradiated uranium generally to an extent up to about 0.02 percent by weight. Because the fission products in general are highly radioactive, it is preferred. that these materials be removed.

The fission products consist of a large number of elements which may be classified into two groups: a light group with atomic numbers from 35 to 45; and a heavy group with atomic numbers from 5l to 60. The fission products with which we are particularly concerned are thosehaving a half-life of more than three days Since they remain in the neutron irradiated reaction mass in substantial quantities at least one month after reaction.

These products are chiefly Sr29, Y (57 day half-life), Zr, Cb, and Ru of the group of atomic numbers from 35 to 45; and "fe/127, Te129, 1131, Xe133, Cs (many years half-life), Ba (l2 days half-life), La140, and Ce of 2() day and 20G day half-lives from the group of atomic numbers from 5 l to 60, inclusive.

Plutonium is found to have at least two states of oxidation. ln its lower state of oxidation the plutonium is substantially insoluble in water as a phosphate or tluoride and is believed to have a valence of 4. In its higher state of oxidation, the plutonium is water soluble as a phosphate or uoride and is believed to have a valence of 6. This oxidized plutonium is capable of forming a plutonyl ion having the probable structure PuOg-HL. Frequently plutonium in its reduced or fluoride insoluble state is designated as Puf and plutonium in its oxidized or fluoride soluble state is designated as Puo.

ln accordance with the present invention it has been found that plutonium (element 94) may be removed from an aqueous solution by precipitation of a complex acetate. For example, when the plutonium is in its higher state of oxidation it may be precipitated as a sodium plutonyl acetate. Moreover it may be carried down from solution by precipitation of sodium uranyl acetate which adsorbs or carries the plutonium from solution. Likewise sodium uranyl acetate may be precipitated or otherwise formed in a linely divided or easily distributable form and this product introduced into the plutonium solution for removal of the plutonium.. lt has been found that plutonium in its lower oxidation state or reduced state will not be precipitated or carried to a substantial degree under similar conditions. Inprecipitating plutonium in its higher oxidation state with an alkali metal as a complex acetate, the plutonium is separated to a substantial degree from fission products since they remain in solution. Moreover the above separation process may be used to separate plutonium in its reduced state from uranium by precipitation of the uranium as an alkali metal uranyl acetate while the plutonium remains in solution. Where both of the above separation processes are used, it is contemplated that they may be carried out as shown in Fig. 2 in which the plutonium and fission products are first separated from the uranium and then from each other, or as shown in Fig. 1l in which the plutonium and uranium are first separated from the fission products and then from each other.

While various suitable oxidizing agents may be used to change the oxidation of plutonium in its reduced `state (Puf) to its higher oxidation state (P), it has been found that dichromate ions (Cr2O7-) are particularly suitable for this purpose. Reducing agents such as sulfur dioxide, hydroxylamine, or hydrogen peroxide have been found to be satisfactory for converting plutonium from its higher oxidation state (PW) to its reduced state (Puf). Usually this reduction is conducted in the preence of a small quantity of acid such as sulphuric acl In separating the plutonium from the fission products by precipitation of the plutonium in yits higher oxidation state as a complex acetate, acetate ions are introduced into the solution in any convenient form, such as by sodium acetate, preferably following the addition of acetic .acid in order to adjust the acetic acid/ acetate ion ratio to such a value that the danger of precipitating any salt other than the desired complex salt is substantially completely eliminated. To precipitate the plutonyl acetate complex, an excess of sodium ions are introduced into the solution in the form of a sodium salt. Preferably, sodium nitrate may be used for introducing sodium ions and sodium acetate for introducing acetate ions. lt is believed that the plutonyl ions combine with the acetate ions to form a complex acetate, the complex being prepicitated by the alkali metal ions in accordance with the following formula:

in which R is an alkali metal such as sodium, potassium or lithium. It has been found that a substantial excess .of sodium ions facilitates the precipitation of the plutonium as a complex acetate.

Where the plutonium is present in the solution in low concentrations and it is desired to precipitate the plutonium in its higher oxidation state, a carrier may be used to aid in taking the plutonium out of the solution. It has been found that uranium in the form of an alkali metal uranyl acetate precipitate may be used for this purpose. The uranium in its higher oxidation state is precipitated as a complex uranyl acetate by alkali met-al ions, such as sodium ions, in a manner similar to that in Which the plutonium is precipitated. Frequently, the uranium is used as a carrier even where the plutonium is present in relatively high concentrations as its use increases the yield of plutonium separated from the fission products.

Where a solution of neutron irradiated uranium is' treated first with the plutonium in its higher oxidation state, the presence of uranium in the solution makes unnecessary the addition of further amounts of uranium for use as a carrier. In the precipitation of the plutonium as a complex acetate, a portion of the uranium is likewise precipitated and by substantial precipitation of the uranium the plutonium is carried out of the solution containing the fission products. Where a solution of neutron irradiated uranium, however, is first treated to precipitate an alkali uranyl acetate with the plutonium in its reduced state so as to obtain a substantially uranium-free solution containing the plutonium and fission products, subsequent precipitation of the plutonium in its higher oxidation state to separate it from the fission product is facilitated by the addition of small amounts of uranium to carry the plutonium from the solution. The addition of uranium as a carrier makes necessary a further separation step if the plutonium is to be obtained free of the uranium. There is an advantage, however, in such addition inasmuch as the resulting precipitate may be dissolved in a small amount of solvent such as nitric acid, thereby greatly increasing the concentration of plutonium in the solution. The addition of small amounts of uranium for concentration of the plutonium may advantageously be employed where the plutonium has been obtained free from fission, products and uranium irrespective of whether the plutonium was first treated in its reduced state` or in its oxidized state.

This process may be conducted to effectively concen-. trate the plutonium by use of a plurality of successive plutonium precipitations. Thus plutonium may bev pre-- cipitated from aqueous solution by precipitation of sodium uranyl acetate, the precipitate redissolved in a smaller volume of acid solution, the plutonium reduced, the uranium separated from the solution leaving the reduced plutonium in solution, the plutonium oxidized and the process repeated a number of times using a progressively smaller quantity of solution to dissolve the uraniumplutonium precipitate and progressively smaller portions of uranium to effect the precipitation.

For example a uranium nitrate solution containing plutonium as Pu may be treated with sodium and acetate ions to precipitate sodium uranyl lacetate and to carry the plutonium down with the precipitate. This precipitate may be dissolved in a minimum of nitric acid and the plutonium reduced to the Puf state by means of a reducing agent such as hydroxylamine hydrochloride or a sulphite such as sodium bisulphite which leaves the uranium in hexavalent state. 1 Thereafter the uranium may be largely precipitated as sodium uranyl acetate and the plutonium reoxidized to the PuJ state. The solution containing the Puo may then be treated with a smaller amount of uranium, usually not over l0 percent of the initial uranium concentration and sufficient sodium and acetate ions supplied to the solution to precipitate the uranium and plutonium. This precipitate may be dissolved in a minimum of nitric acid and the process repeated until the desired concentration of plutonium has been secured.

The following examples are illustrative:

Example 1 Uranyl nitrate hexahydrate containing 0.00000005 percent Pu by weight was dissolved in water and nitric acid added so that the resulting solution contained 0.55 mole per liter of UO2(NO3)26H2O and 0.87 mole per liter of HNO3. To this solution was added sufficient sodium dichromate, NaZCrZOf, to make the solution v0.087 mole per liter in dichromate ion. This was then heated at 75 C. for 30 minutes `after which solid sodium acetate was added in such amount as to give a ratio of 6 moles of NaAc.3H2O to every mole of UO2(NO3)2.6H2O. v

This caused the uranyl ion (UO2++) to be precipitated as the insoluble sodium uranyl acetate and the plutonyl ion PuO2++ to be precipitated probably as sodium plutonyl acetate or at least to be carried out of solution by the precipitation of sodium uranyl acetate. The 'precipitate was removed by filtration and washed with a solution which was about 0.5 mole per liter in NaAc.3H2O, 0.5 mole per liter in HAc and 1.5 moles per liter'in NaNO3. The filtrate contained substantially all of the ssion products and negligible amounts of plutonium and uranium.

The mixed complex acetate was dissolved'in nitric acid to give a solution which was 0.87 mole per liter in UO2++ and 3.4 moles per liter in HNOB (mole ratio of nitric acid to UO2++ was 4 to l). Sodium acid sulphite in such amount as to give a mole ratio of l mole of NaI-1803 to 5.5 moles of UO2++ was then added to bring about reduction of the Pun to Puf. This reduction is complete in about l0 minutes. The UO2++ was again precipitated leaving Pulr in solution by adding NaAc.3H2O in such quantity as to give a mole ratio of 6 moles of NaAc.6H2O to one mole of UO2++ ion. The precipitate of sodium uranyl acetate was removed by filtration and was washed` with a solution which was 0.5 mole per liter in NaAc.H2O, 0.5 mole per liter in HAC and 1.5 moles per liter in NaNO3. Over percent of the plutonium remained in solution in the ltrate which contained but negligible amounts of fission products and uranium.

Example 2 A solution of uranyl nitrate hexahydrate containing 0.00000005 percent Pu by weight was adjusted to be 0.55 M in UO2++ ion and 0.2 M in H+ (HNOS) ion. This solution was treated with hydroxylamine hydrochloride -so that it is 0.1 M in NH3OH+ ion to put the Pu in the reduced form Pu]E ion leaving uranium substantially unreduced. NaAc.3H2O was then added in quantity sucient to give a Na+ concentration of 1.5 M. Sodium uranyl acetate then precipitated from this solution. The

precipitate was washed with a small volume ot wash solution made up of NaNO3, NaAc.3H2O, and HAc in such proportion as to have the following concentrations: Na+ 1.5 M, Ac- .2 M, and HAc.2 M. The wash water was added to the filtrate containing the reduced Pu. The filtrate and wash was then treated with nitric acid and sodium dichromate in such quantity as to make the solution 0.2 M and H+ (HNOS) and 0.1 M. in Cr2Oq= ion. After heating for about 30 minutes at 75 C. the Pu was in the higher state of valence, namely PuO2++ ion. Uranyl nitrate hexahydrate was added to the extent of 1/ 0 of the amount originally present and this solution was treated with NaNO3 and NaAc.3H2O in such quantity as t-o maintain concentrations in the supernatant solution of Na+ 5.0 M, Ac- 0.2 M, and HAc 0.5 M, whereby the complex uranyl and plutonyl acetates are precipitated. The NaNO3 was used in order to raise the Na ion concentration level to a high value in order to ensure substantially complete precipitation of the plutonyl complex. The precipitate was filtered out and washed with two small portions of a wash containing NaNO3, NaAc.3H2O, and HAC in the following concentrations: Na+ 5.0 M, Ac` 0.2 M, and HAC 0.35 M, and both filtrate and wash were discarded. The iiltrate and Wash, containing substantially all of the fission products and negligible amounts of plutonium, was discarded.

The precipitate contained substantially all of the plutonium, the added uranium, and 0.17 percent of the fission products. The precipitate was dissolved in a minimum of nitric acid as in Example l, and hydroxylamine hydrochloride (NHgOHCl) was added to reduce the plutonium. An excess of sodium acetate was added to precipitate the uranium as sodium uranyl acetate as above described. The precipitate was removed from the solution by filtration and washed with three 50 ml. portions of wash solution containing 1.5 M Na+, 0.2 M Ac, and 0.2 M HAC. The precipitate containing substantially all of the added uranium and negligible amounts of plutonium and iission products was discarded.

The filtrate and wash contained the remainder of the plutonium and a small percentage of the fission products. The plutonium was again precipitated with sodium acetate and uranyl nitrate as before using about 1&0 of the amount of uranium previously used. The major portion of the plutonium was collected in the precipitate while the iis sion products largely remained in solution.

Where the negligible amount of fission products at the end of the lirst cycle is not objectionable, the precipitate may be further treated to separate the plutonium from the uranium that was added for use as a carrier. In such case, the precipitate may be dissolved in nitric acid and the plutonium reduced with NH3OHC1. An excess of sodium acetate may then be added to precipitate the uranium while maintaining the plutonium in solution. The sodium uranyl acetate may be removed from the solution by filtration.

Where the concentration of plutonium in the solution has reached fairly high levels and it becomes desirable to separate the plutonium and uranium, this may be done by removing the uranium as sodium uranyl acetate while maintaining plutonium inthe reduced state in solution. The plutonium if in sulicient concentration may be obtained in solid form by being oxidized and precipitated as a complex acetate without the addition of uranium. The plutonium may be obtained in solid form also by evaporating the solution containing the reduced plutonium or by the use of a precipitant that converts the plutonium to an insoluble plutonous compound such as a phosphate, fluoride, or the like.

Example 3 0.43 microliter of a solution of uranyl nitrate hexahydrate containing 0.0413 microgram of Pu, 117 micrograms of uranium containing 0.1 mol per liter of NazCrzOq and 0.1 mol per liter of HNO3 was heated at 6 130 C. for l hour in a sealed capillary, sodium acetate and nitric acid were added and the volume was adjusted to form a solution containing 0.4 mole per liter of HAc, 0.2 mole per liter of acetate ion and 1.5 moles per liter of sodium ion and suflicient water was added to increase the volume of the solution to one microliter.

The precipitate was separated by centrifugation and washed with three `portions of water each portion being about 0.14 microliter in volume.

The precipitate was dissolved in nitric acid to form a solution containing about 0.2 mole per liter of nitric acid and 0.55 mole per liter of UO2++ and hydroxylamine hydrochloride was added in amount suiiicent to establish a concentration of 0.1 mole per liter of NH3OH+. After the solution was all-owed to stand sodium acetate and acetic acid were added in amount suiiicient to form a solution 0.9 microliter in volume and containing 0.2 mole per liter of acetate ion, l.9 moles per liter of HAC, 1.5 moles per liter of sodium ion and 0.1 mole per liter of NH3OH+- The precipitate was removed and washed with three portions of water each 0.14 microliter in volume. The combined wash water and the filtrate contained 89 percent of the plutonium and negligible amounts of fission products and uranium.

T he precipitation method, as it is generally carried out, separates element 93 as well as plutonium from the foreign substances, such as uranium, fission products, and the like. Since element 93 undergoes beta decay to plutonium with a half-life of only 2.3 days, the process may be carried out with only a negligible amount of element 93 present if the reaction mass is stored for a suitable length of time. Thus, in approximately sixteen days over 99 percent of element 93 is converted to plutonium and at the end of approximately thirty days over 99.9 percent of element 93 has been converted to plutonium through beta decay. v

Where it is desired to proceed without storing the neutron irradiated uranium for substantial conversion of element 93 to plutonium by beta decay, element 93 may be separated from plutonium by taking yadvantage of the different oxidation characteristics of the two elements both of which possess a higher and a lower valent state. In an example of such separation, the neutron irradiated uranium is dissolved as by nitric acid and element 93 and plutonium are maintained or converted to their reduced state 'by the addition of a reducing agent such as sulphur dioxide. Potassium 'bromate may then be added to the solution to oxidize element 93 while permitting the plutonium to remain in its reduced state. It has been found that the addition of 0.5 gnam of potassium bromate to 30 cc. of solution containing tracer amounts of element 93 and plutonium will oxidize all of the element 93, leaving 94 unoxidized provided the solution is permitted to stand at room temperature for not longer than thirty minutes land provided no cerium is present. Ceriurn` which may be present as a fission product should lbe removed before such a separation is attempted. Where ele ment 93 has been obtained in its oxidized state with plutonium remaining in its reduced state, the two may be separated by precipitants such by precipitation of sodium uranyl acetate in the solution whereby the neptunium is carried with the precipitate probably as alkali metal neptunyl acetate, i. e., sodium neptunyl acetate. Where the concentration of neptunium is sufficiently high, sodium neptunyl acetate or its equivalent may be precipitated without addition of uranium to form sodium uranyl acetate. In accordance with a further modification, the

solution may be treated with phosphate ions which will precipitate plutonium while permitting element 93 to remain in solution. The particular precipitate obtained can be separated from the solution by any conventional means such as filtration, centrifugation, or the like.

In accordance with afurther modification of the invention other plutonium salts particularly plutonyl salts of organic carboxylic acids may be prepared. Thus while the invention is particularly adapted to the recovery of plutonium from aqueous media by formation of `an alkali metal plutonyl acetate, the process may also be used for production of other plutonium salts such as sodium plutonyl propionate, sodium plutonyl mono-chloracetate or salts of other organic acids such as chloracetic acid,v propionic acid, butyric'acid, etc.

The above detailed description is given forpurposes of illustration and the invention is to be limited only by the scope of the appended claims.

We claim:

l. In a process for the separation of plutonium from uranium, the step which comprises contacting `a solution containing uranium and plutonium in their hexavalent states and ssion products with an alkali metal acetate whereby the uranium is precipitated as an alkali metal uranyl acetate which carries the plutonium therewith.

2. Process of claim 1 in which alkali metal acetate is sodium acetate.

3. In a process for the separation of plutonium from uranium, the steps which comprise vcontacting a solution of neutron irradiated uranium wherein the uranium is in a hexavalent state of oxidation and the plutonium is in a state of oxidation not greater than 1-4 with an alkali metal iacetate to precipitate the uranium as the corresponding alkali metal uranyl acetate, removing the latter from the solution, oxidizing the plutonium to the hexavalent state, and thereafter contacting the solution containing the oxidized plutonium with an alkali metal acetate to precipitate the plutonium as the corresponding alkali metal plutonyl acetate.

4. In a process for separating plutonium from uranium, the step which `comprises contacting a solution containing uranium in the hexavalent state of oxidation and plutonium in a state of oxidation not greater than |4 with :an alkali metal acetate whereby the uranium is precipitated from the. solution as an alkali metal uranyl acetate.

5. The process of claim 3 in which the solution of neutron irradiate uranium is prepared by dissolving the latter in nitric acid.

6. A method of concentrating plutonium which comprises contacting an aqueous solution containing hexavalent plutonium ions with4 a substantially insoluble alkali metal uranyl acetate whereby the plutonium becomes lassociated with the'alkali metal uranyl acetate precipitate, removing the precipitate and thereafter dissolving the precipitate in acid to produce a solution more concentrated than the original with respect to plutonium, reducing the latter, contacting the resulting solution with an alkali metal acetate to form the corresponding insoluble alkali metal uranyl acetate, oxidizing the dissolved 8 plutonium to the hexavalent state and thereafter contacting the resulting solution with a smaller quantity of an @alkali metal uranyl acetate to remove thev plutonium in a more concentrated form.

7. The process of claim 6 in which the alkali metal uranyl acetate is sodium uranyl acetate and the alkali metal acetate is sodium acetate.

8. In a method for the separation of plutonium from neptunium, the steps Which comprise forming a solution containing neptunium in its hexavalent state of valence while maintaining the plutonium in a valence state not greater than +4 and contacting said solution with an alkali metal acetate.

9. The process of claim 8 in which the alkali metal acetate is sodium acetate.

l0. A method of separating plutonium from uranium where the plutonium is present in very low concentrations comprising forming an aqueous nitric acid solution thereof, reducing the plutonium to a valence state not greater than +4, contacting the solution with an alkali metal acetate to form insoluble alkali metal uranyl acetate, separating the 'alkali metal uranyl acetate from the solution, oxidizing the plutonium to the +6 valence state, adding uranium nitrate to the solution, the amount of uranium added `being much less than that precipitated from the solution in the preceding stage, contacting the solution with an alkali metal acetate to form insoluble alkali metal uranyl acetate and alkali metal plutonyl acetate, separating these acetates from the solution, reducing the plutonium to a valence state not greater than |4, contacting ,the solution with an alkali metal acetate whereby the uranium is precipitated as insoluble alkali metal uranyl acetate, and separating the alkali metal uranyl acetate.l

References Cited in the file of this patent UNITED STATES PATENTS Fermi et al. Oct. 3, 1935 OTHER REFERENCES 

10. A METHOD OF SEPARATING PLUTONIUM FROM URANIUM WHERE THE PLUTONIUM IS PRESENT IN VERY LOW CONCENTRATIONS COMPRISING FORMING AN AQUEOUS NITRIC ACID SOLUTION THEREOF, REDUCING THE PLUTONIUM TO A VALENCE STATE NOT GREATER THAN +4, CONTRACTING THE SOLUTION WITH AN ALKALI METAL ACERATE TO FORM INSOLUBLE ALKALI METAL URANYL ACETATE, SEPARATING THE ALKALI METAL URANYL ACETATE FROM THE SOLUTION. OXIDIZING THE PLUTONIUM TO THE +6 VALENCE STATE, ADDING URANIUM NITRATE TO THE SOLUTION, THE AMOUNT OF URANIUM ADDED BEING MUCH LESS THAN THAT PRECIPITATED FROM THE SOLUTION IN THE PRECEDING STAGE, CONTACTING THE SOLUTION WITH AN ALKALI METAL ACETATE TO FROM INSOLUBLE ALKALI METALL URANYL ACETATE AND ALKALI METAL PLUTONYL ACETATE, SEPARATING THESE ACETATES FROM THE SOLUTION, REDUCING THE PLUTONIUM TO A VALENCE STATE NOT GREATER THAN +4, CONTACTING THE SOLUTION WITH AN ALKALI METAL ACERATE WHEREBY THE URANIUM IS PRECIPITATED AS INSOLUBLE ALKALI METAL URANYL ACETATE, AND SEPARATING THE ALKALI METAL URANYL ACETATE. 